X-RAY DIFFRACTION STUDIES ON HEAVY-METAL FERROCYANIDE GELS' HARRY B. WEISER, W. 0. MILLIGAN, ASD J. B. BATES Department of Chemistry, The Rice Institute, Houston, Texas Received July
14,1058
Most heavy-metal ferrocyanides are so insoluble that they are thrown down in a highly gelatinous form on mixing even dilute solutions of metallic ion and ferrocyanide ion. The tendency of the gel to carry down ferrocyanide ion is so strong that the precipitation of the metal is incomplete on mixing equivalent amounts of metallic and ferrocyanide ions. This strong sorption is illustrated by the carrying down of potassium ferrocyanide, sodium ferrocyanide, and hydroferrocyanic acid during the precipitation of copper ferrocyanide (figure 1) (7, 11). Some of the heavymetal ferrocyanide gels thrown down with excess ferrocyanide ion are assumed to be double salts such as 2CupFe(CN)s.K4Fe(CN)o. From the potassium ferrocyanide curve in figure 1, it might be argued that a definite double salt is formed having the formula 5CuzFe(CN)o.2&Fe(CN)e, and that the upper portion of the curve represents the sorption of potassium ferrocyanide by the double salt. Although this is possible, the evidence is not sufficient to establish the existence of a definite double salt of this formula. Attempts have been made to determine the composition of the precipitated ferrocyanide gels by electrometric (I, 8, 9) and conductometric (4,5, 6) titration. The most recent work of this kind was done by Britton and Dodd (21, who made conductivity measurements a t 25OC. on mixtures of heavy-metal salts and potassium ferrocyanide: first, in the form of direct conductometric titration of 125 cc. of 0.02 M salts with 0.1 M potassium ferrocyanide; and second, on similar mixtures of reactants which had stood in a thermostal until equilibrium was set up. In figure 2 are given the curves constructed from data corresponding to equilibrium conditions and the horizontal lilies which represent the specific conductivity of potassium sulfate formed as a result of the equation: 2MS04
+ (1 + z)KaFe(CN)c-tM2Fe(CN)o.zK4Fe(CN)6+ 2KzSO4
Presented a t the Fifteenth Colloid Symposium, held a t Cambridge, Massachusetts, June 9-11, 1938. 945
946
HARRY B. WEISER, W . 0 . MILLIQAN, AND J . B. BATES
The values of z [ = K4Fe(CXl\r] deduced from the conductivity data, and from analysis of the precipitates formed in the presence of varying excess of potassium ferrocyanide, are summarized in table 1 (Britton and Dodd). The direct analysis of the precipitates is of little use in estimating the exact composition, since they are peptized before they can be washed free from entrained salt. Hence the values in the last column of table 1 represent only very rough approximations.
I
1
FIG.1. Sorption of salts by copper ferrocyanide gel
Britton and Dodd recognize that the conductometric and analytical evidence given above is insufficient to decide whether the M2Fe(CK)sK4Fe(CN)e mixtures are definite double salts or sorption complexes. Moreover, since any highly gelatinous double salt that may form will sorb ferrocyanide, it is not possible from conductometric data to deduce the ratio of M2Fe(CX)6to &Fe(C?;)s in the alleged double salts. Since Nilligan (10) found that the gel of copper ferrocyanide i s crystalline to x-rays, it seemed likely that the ferrocyanide gels of other metals
HEAVY-METAL FERROCYANIDE GELS
947
FIG.2. Titration curves for ferrocyanide solution s.itli various metallic salts (Britton and Dodd) TABLE 1 ic
Potassium ferrocyanide i n jerrocyanide gels = moles of KdFe(CN)a per mole of ;\12Fe(CS)R FROM COPI'DUC'IIVITY MEASCREMEXTS
M
First break
Second break
0.1 .1f
FROM DIRECT ATALYSIS 7
~~
CC.
CU . . . . . . . . . . . . . . . . Zn . . . . . . . . . . . . . . . . Cd ....................... Si . . . . . . . . . . . . . . . ..... Mn . . . . . . . . . . . . . . . . . . . . . . . . . . Pb . . . . . . . . . . . . . . . . . . . . . . . . . . Ag . . . . . . . . . . . . . . . . . . . . . . .
co.
I
-____
0 52-0.71 0.71-0.78 0.59-0.90 0.50-0.74 0.56-1.22 0.83-0.92 Very small 0.33
948
' . 0 . MILLIGAN, AND J. B. BATES HARRI- B. WEISER, U
,
I f
,
949
HEAVY-METAL FERROCYANIDE GELS
would be crystalline. If so, the method of x-ray diffraction analysis should throw some light on the constitution of the gels formed under varying conditions. This paper gives a preliminary report of the results of the x-ray examination of the ferrocyanide gels of copper, cobalt, nickel, manganese, lead, zinc, cadmium, and silver. EXPERIMENTAL
Copper ferrocyanide In an investigation of the cause of the impermeability of the copper ferrocyanide membrane to ferrocyanide ion (ll),it was concluded that the salt sorbs ferrocyanide ion strongly and irreversibly up to a composition of approximately 0.4 mole of potassium ferrocyanide per mole of copper ferrocyanide (figure 1). But, as already pointed out, the possibility of double salt formation can be neither proven nor ruled out by the results TABLE 2 A series of copper ferrocyanide gels AYOUNT OF
BOLUTIONS MIXED
0.0874 M
0.0300 M
0.1173 AM
0.0804 M
cc.
cc.
CC. 197.92
cc.
-I I1 I11 IV V VI VI1
&Fe(CN)s ADOORRED,
K,Fe(CN)s
CUCll
BAYPIE NO.
802.88 100.36 100.36 100.36 100.36 100.36 100.36
MOLES PER MOLE OF
CurFe(CN)s CC.
0
39.73 49.48 54.50 59.34 61.84 64.36
108.91 99.19 94.17 89.33 86.81
miUimoles
24.88 2.55 0.305 0 0 0
moles
0.022 0.037 0.042 0.117 0.224 0.268 0.319
recorded in figure 1. A series of gels was therefore prepared by mixing varying amounts of cupric chloride and potassium ferrocyanide as given in table 2. The gels were centrifuged and the supernatant liquids analyzed for copper and ferrocyanide. The copper determinations were made by the standard iodometric procedure, while the ferrocyanide analyses were carried out by titrating with standardized potassium permanganate. It will be observed that the first three samples in the table contain but little excess potassium ferrocyanide. A duplicate of sample I was washed, using a centrifuge, until the last washing was free of copper and ferrocyanide ions. This gel was air-dried and analyzed by a method similar to that of Hartung (3). The dry gel was dissolved in 6 N sodium hydroxide, and boiled until copper oxide was precipitated. The filtered and washed copper oxide was dissolved in nitric acid, excess acid removed, and the copper determined as given above. The filtrate was neutralized and ferrocyanide determined by
950
WRRY
w.
B. WEISER,
0. MILLIOAN, AND J. B. BATES
titration with potassium permanganate. The sample was found to contain 41.53 per cent Cu, 24.42 per cent Fe(CN)e, and 34.05 per cent HzO (by difference); the mole ratio Cu/B'e(CN)s = 1.962. The unwashed, moist gels obtained by centrifuging samples I to VI1 were placed in thin tubes of Lindemann glass for x-ray examination by means of the following procedure: The open end of the glass tube was thrust into a portion of the moist gel placed on a sheet of filter paper. The small quantity of gel obtained was moved toward the center of the glass tube by gentle suction, leaving about 4 to 5 mm. of empty tube below the
I
IIIII
I
I
I1I
I
I
I
PER MOLE
I
C",F*(cN)6
0042 Oo3'
0 268 / / / / 1 1 1
0319 CuzFe(C& AIR-DRIED
GEL FROM K,F.[CN)& CG*(C% AIR-DRIED
GEL FROM H4Fe(CN)b
0
I
2
3
4
5
6
7CM
FIG.4. Diagrams of the x-ray diffraction patterns of copper ferrocyanide gels containing varying amounts of sorbed potassium ferrocyanide.
gel. The Lindemann glass tube was then sealed off by means of a very small needle-point flame. The various gel samples were examined by x-ray diffraction methods, using Cu K, x-radiation, in a camera 57.6 mm. in diameter, with an exposure time of 1 hr. The film was protected by aluminum foil from secondary radiation from the iron in the samples. Some typical patterns reproduced in figure 3 show the degree of crystallinity of the gels. The results with a series of gels of varying composition are represented diagrammatically in figure 4. For purposes of comparison the diagrams of practically pure copper ferrocyanide precipitated (a) with copper in excess and ( 6 ) with hydroferrocyanic acid in slight excess are included in the diagram. Since
HEAVY-METAL FERROCYANIDE GELS
95 1
the several patterns are identical within the limits of accuracy of the observations, it would appear that the ferrocyanide carried down by copper ferrocyanide gel is not combined to form a definite double salt. There are three possibilities: (1) that the excess ferrocyanide is adsorbed on the surface of the highly dispersed crystals, (2) that the excess ferrocyanide is dissolved in the normal salt without causing sufficient change in the diffraction pattern to be detected, and (3) that both adsorption and solid solution are involved in the process. Since the crystals are so minute, x-ray diffraction patterns of the gels are not sufficiently sharp to enable one to detect minor differencesin posibion or intensity of the bands. Until further information is available it seems advisable to use McBain’s term “sorption” in referring to the phenomenon, whichmay involve both adsorption and a small amount of solid solution.
Other ferrocyanides To prepare the other ferrocyanide gels investigated, 125-cc. portions of 0.02 M solutions of the several salts were added to varying amounts of 0.1 M potassium ferrocyanide (cf. Britton and Dodd (2)). Each ferrocyanide was precipitated (1) with one-half the theoretical amount required to react with the metal, (3) with an amount corresponding approximately to the break in Britton and Dodd’s titration curve, and (3) with three times the theoretical amount. The gels were thrown down with the centrifuge and the x-ray diffraction patterns obtained as described above for copper ferrocyanide. The ferrocyanides of cobalt, nickel, and manganese. The x-ray diffraction patterns of the several gels precipitated under varying conditions are shown diagrammatically in figure 5. With all three salts the diffraction patterns of the gels are the same irrespective of the K4Fe(CN)e:2MzSO4 ratio used in their preparation. This indicates that the potassium ferrocyanide carried down by the several gels is sorbed in an indefinite ratio and is not combined to give definite double salts. The breaks in the conductometric titration curves are therefore without significance in their relation to the possible formation of double salts. They may correspond approximately to the flat portion of the respective sorption isotherms. A comparison of the diffraction patterns of the several ferrocyanides under consideration discloses that the positions of the lines are the same within the limits of the accuracy of measurement. The fairly sharp diffraction lines are in the same position, and the number and intensity of the lines are the same for the four salts, as nearly as can be estimated. This indicates that the salts are isomorphous, and have lattice constants that are nearly the same. This would be expected if the relatively large ferrocyanide anions were so arranged as to give spaces in which the smaller metallic ions are grouped. The fact that the ionic radii of the metallic ions under consideration are very nearly the same would account for
952
HARRY B . WEISER, W. 0. MILLIGAN, AND J. B . BATES
the apparently small variations in lattice constants among the several salts. Lead ferrocyanide. The lead salt gives a precipitate composed of relatively large crystals with a low sorbing power. The diffraction pattern shown in figure 5 is different from that of the other salts in the same figure. This would be expected in view of the much larger ionic radius of lead ion. The low sorption capacity accounts for the break in the conductometric titration curve a t a point only a little above that corresponding to equivalent amounts of lead and ferrocyanide ions. 3
2
I
4
5
6
7CM
MMED
S -O /LUT O INS
FI(C~@I~ M"
]
04
I 18 24
04 I 8
24 04
il," 2 4
04
24
FIG.5, Diagrams of the x-ray diffraction patterns of the ferrocyanide gels of cobalt, nickel, manganese, and lead. T h e ferrocyanides of z i n c , c a d m i u m , and szlver. The diffraction patterns of the ferrocyanides of zinc, cadmium, and silver are sho.c\n in diagrammatic form in figure 6 . In contrast to the salts whose diffraction patterns
are given in figure 5, the zinc, cadmium, and silver salts each give two patterns, depending on whether the gel is thrown down with metallic ion in excess or with ferrocyanide ion in excess. The two diffraction patterns for each of the three salts may correspond respectively to the normal salt and to a double salt with potassium ferrocyanide. It should be pointed out, however, that the possibility has not been excluded that the second pattern is due to a different crystalline modification or a different hydrate of the normal salt plus sorbed potassium ferrocyanide. The composition of zinc ferrocyanide is of special importance, because
HEAVY-METAL FERROCTANIDE GELS
I
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/ I 1
I l l
I
I
I
I
/ I
I I
I /
1
1
I, I ,
I
I
I
.
.
l l l l l l l
/I Ill,
I
I
F.(CN)~;
Cu, Fe(CNj6
2"
04
I3
I Ill
I/
Zn
I
I
I
953
FIG.6. Diagrams of the x-ray diffraction patterns of the ferrocyanide gels of zinc, cadmium, and silver.
in the composition of the precipitated gel and so to obtain accurate results in the estimation of zinc, it is essential not only that the conditions be rigidly controlled but also that they be exactly the same as in the standardization of the ferrocyanide solution against zinc. Attention should be called to the fact that the diffraction pattern of the cadmium salt thrown down with ferrocyanide in excess is very similar to that of copper ferrocyanide. This suggests that the cadmium salt formed under these conditions is CdzFe(CN)O. SUMMARY
The results of this investigation may be summarized as follows: 1. The gels of the heavy-metal ferrocyanides carry down alkali ferrocyanides strongly. It is not possible to determine by conductometric or potentiometric titration whether the gels adsorb the alkali ferrocyanide or whether double salts are formed.
954
HARRY B. WEISER, W. 0. MILLIGAN, AND J. B. BATES
2. Since the heavy-metal ferrocyanides are crystalline to x-rays, x-ray diffraction analysis of the moist gels throws some light on the constitution of the gels formed under varying conditions. 3. Copper ferrocyanide precipitated in the presence of excess copper is practically pure Cu&'e(CN)6, which gives a distinctive x-radiogram. The gel thrown down with excess alkali ferrocyanide gives the same x-ray diffraction pattern as cu~Fe(CN)6even when it contains as much as 0.3 to 0.4 mole of K4Fe(CN)6per mole of c~&'e(cN)~. 4. The alkali ferrocyanide carried down by copper ferrocyanide gel is not combined to form definite double salts. The excess ferrocyanide is adsorbed, for the most part, on the surface of the highly dispersed crystals of copper ferrocyanide. The possibility that a small amount of alkali ferrocyanide is dissolved in copper ferrocyanide has not been excluded. The phenomenon is therefore referred to as sorption. 5. The ferrocyanides of copper, cobalt, nickel, and manganese, but not of lead, are isomorphous, with lattice constants that are nearly the same. The crystals of lead ferrocyanide are relat'ively large and have a relatively low adsorption capacity for ferrocyanide ion. 6. The ferrocyanides of zinc, cadmium, and silver each give two distinct x-ray diffraction patterns, depending on whether the gel is thrown down with metal in excess or wibh potassium ferrocyanide in excess. The two diffract,ion patterns for each of the three salts may correspond respectively to the normal salt and t'o a double salt with potassium ferrocyanide. The similarity of the diffraction pattern of the cadmium ferrocyanide gel, thrown down with ferrocyanide in excess, to that of copper ferrocyanide suggests that the cadmium sa,lt formed under these conditions is Cd2Fe(CN)6. 7. The breaks in the conductometric titration curves for metallic salts and alkali ferrocyanide are without significance in t'heir relation to the possible formation of double salts. They may correspond approximately to the flat portion of the respective sorption isotherms. REFEREXCES (1) BICHOWSKY: Ind. Eng. Chem. 9, 668 (1917). AND DODD:J. Chem. Soc. 1933,1543. (2) BRITTON (3) HARTUNG: Trans. Faraday Soc. 16, Pt. 3, 160 (1920). (4) Cf. IBARZ A N D FEYTO:Anales soc. espail. ffs. quim. 34, 823 (1936). (5) KOLTHOFF:Z. anal. Chem. 62, 209 (1923). (6) KOLTHOFF AND VERZIJL:Rec. trav. chim. 43,394 (1924). (7) MULLER,~TEGELES, AXD KELLERHOFF: J. prakt. Chem. [2] 86, 82 (1912). (8) TREADWELL .LXD CHERVET: Helv. Chim. Acta 6, 633 (1922); 6, 550 (1923). (9) TREADWELL AND WEISS: Helv. Chim. Acta 2, 680 (1919). (10) Cf. WEISER:Inorganic Colloid Chemistry, Vol. 111, p. 309. John Wiley and Sons, Inc., Xew York (1938). (11) WEISER:J. Phys. Chem. 34, 335 (1930).
